Copying Apple's system is now an Apple-proprietary endeavor; we can only offer "best effort" support for making an external bootable device on macOS Big Sur (and later OSes). We present this functionality in support of making ad hoc bootable copies of the system that you will use immediately (e.g. when migrating to a different disk, or for testing purposes), but we do not support nor recommend making bootable copies of the system as part of a backup strategy.